Mane wouldn't be zero on the books.
£34m initially over five years = £6.8m per season. If he signed a new deal after three years; he'd be £13.6m in the accounts at that point. If he signed a three year deal, that £13.6m is split over those three years (£4.5m per season).
If he were to leave in the final year of that second contract, he'd be on the books as a £4.5m asset. Pocket changed really but not zero.
I don't know when the contract was renewed etc so the above is just an example. Theatrically, can never be zero.